Late-onset intracranial adenocarcinoma arising from a germ cell tumor 25 years after initial diagnosis.

A teratoma with somatic-type malignancy is a rare subtype of germ cell tumor (GCT) characterized by the presence of a histologically distinct component resembling a somatic cancer within a teratoma feature. To date, only 14 cases have been reported in the central nervous system (CNS), and its diagnosis remains challenging due to the limited availability of pathological findings. A case of a pineal GCT that recurred as an adenocarcinoma 25 years after the initial chemoradiotherapy is reported. Although the histological features of teratoma were not observed, transformation into a teratoma with somatic-type malignancy was suspected. DNA methylation analysis classified the tumor within the CNS GCT cluster, specifically aligning with teratomas. Furthermore, whole-exome sequencing demonstrated multiple somatic mutations including two MAPK pathway gene alterations. These findings support the hypothesis that a GCT can evolve into a somatic cancerous phenotype over time, providing deep biological insights into the carcinogenesis of teratomas.